The National Heart, Lung and Blood Institute established clinical practice guidelines for asthma. The clinical effectiveness of these guidelines has not been adequately studied in children during the acute exacerbation of asthma. The focus of this research was to assess the compliance of an emergency department with these guidelines and evaluate the benefit to adherence with an evaluation of clinical, functional and economic outcomes. In addition, the use of an observation unit as an alternative to admission was also evaluated. During year 1 (1998-1999), data collection was completed with a total enrollment of 369 patients. Sample size was adjusted during interim analysis due to effective follow-up of patients (87%), as well as a modification in use of an instrument to measure functional outcome.